Although it is well established that many aspects of visual information processing show decline with advancing age, it is important to conduct research that focuses on identifying the mechanisms associated with preserved visual information processing performance during the adult years. The theoretical framework of the proposed research maintains that expert visual processing performance depends on the efficient use of domain-relevant visual knowledge. The aim of the prop6sed studies is to examine two attentional mechanisms, search efficiency and priming, that are assumed to underlie visual knowledge use in the selected skill domain. Search efficiency acts to confine attention to critical information, and priming mechanisms serve to make critical information more clear, or more quickly or completely encoded. It is suggested that the attentional demands of information selection in complex visual tasks are different, depending on the observer's prior domain-relevant knowledge. Normal age-related limitations should be observed when item-level visual information is not easily extracted. However, such limitations should not be observed for individuals who have specialized knowledge of the item-level units that comprise critical information in the selected domain. The eight proposed experiments are designed to test these predictions by comparing the performance of medical laboratory technologists varying in age (young, middle-aged, older adults) with matched controls on a range of domain-related visual search tasks. Stimulus materials for the proposed studies consist of gram stain photomicrographs as display fields, and diagnostically informative cell types and perceptually comparable items as targets and probes. The proposed research is expected to show that older skilled subjects use domain-specific knowledge to maintain fast/accurate visual information processing performance.